This invention relates to solenoids and in part to solenoids adapted to operated spool-type hydraulic valves or the like in which the hydraulic fluid is permitted to enter into the plunger cavity of the solenoid. Such devices are commonly known in the trade as "wet plunger" solenoids.
Wet plunger solenoids have a distinct advantage over dry solenoids in the operation of spool-type valves and the like in that since the armature or plunger cavity is filled, no dynamic seal is employed between the plunger shaft and the housing, thus eliminating a source of friction during operation and further eliminating a source of possible leakage. However, in the past it has proven difficult to provide a sealed plunger or armature cavity which is, at the same time, a high pressure container. The walls of the plunger cavity have commonly been made as parts which are separate from the fluid seal, thus increasing the effective air gap. Further, wet plunger solenoids in the past have generally operated directly from alternating current and have been designed to operate in either of two limit positions and have not been designed for proportional actuation due to the difficulty of providing the required proportional pole pieces in a sealed cavity.
The invention also relates to a dry solenoid construction according to the above principles.